A wireless LAN has been paid attention as a system which releases users from LAN wirings of a cable scheme. Since the wireless LAN can omit almost all wired cables in a work space such as an office, communication terminals such as personal computers (PC) can be moved relatively easily. Introducing a personal area network (PAN) among others has been studied for information communications by configuring a small scale wireless network of a plurality of electronic apparatus existing around persons. For example, different wireless communication systems and wireless communication apparatus have been prescribed which utilize frequency bands whose licenses by a supervisory office are not necessary, such as a 2.4 GHz band and a 5 GHz band.
Wireless LANs are recently prevailing considerably, because a wireless LAN system becomes inexpensive and is built in PC as a standard. A wireless LAN function in the form of a built-in or external connection adapter is being mounted on a portable apparatus such as a mobile phone and a video camera, in addition to information apparatus such as a PC and a PDA. An example of applications is to upload image data taken with a mobile phone having a camera or a digital camera to a PC via a wireless LAN.
FIG. 5 shows an example of image transmission in a conventional wireless LAN. In FIG. 5, a mobile apparatus is assumed to be a digital camera 100.
A wireless LAN card 101 is mounted on the digital camera 100. The wireless LAN module 101 mounted on a mobile apparatus of this kind is supplied generally in the form of a PC card interface, a compact flash interface or the like. There is a plurality of wireless LAN standards. Many mobile series adopt the standards called 802.11b standardized by IEEE (Institute of Electrical and Electronic Engineers). IEEE802.11b is the wireless LAN standards using a 2.4 GHz band, adopting a DS-SS (Direct Sequence Spectrum Spread) as a modulation scheme, and having a maximum transmission rate of 11 Mbps (for example, refer to Non-Patent Document 1). For example, CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is used as an access control method for IEEE802.11b.
The digital camera 100 stores a photographed image in a built-in memory or an external memory card as image data. For wireless transmission, objective image data is read from the built-in memory or external memory card, and transferred to an image reproducing apparatus such as a PC 102, a television 104 and a printer 106 via the wireless LAN module 101. Obviously, receiving apparatuses side such as the PC 102, the television 104 and the printer 106 are equipped with wireless LAN modules 103, 105 and 107 as adapters. Afterimage data transfer through the wireless LAN, the PC 102 displays and stores the image data, the television 104 displays and outputs it on the screen, and the printer 106 prints and outputs it.
FIG. 6 schematically shows the structure of a conventional digital camera having a wireless LAN function. Reference numeral 200 represents a digital camera having a wireless LAN function. A digital camera itself is constituted of a camera unit 202, a signal processing unit 203, a memory card interface unit 204, an operation/display unit 205 and a USB (Universal Serial Bus) interface unit 206.
The signal processing unit 203 converts image data input from the camera unit 202 into image data having a predetermined format such as JPEG (Joint Photographic Experts Group) and stores the image data in an external memory card 207 via the memory card interface unit 204.
The operation/display unit 205 performs image display, various settings and the like. The USB interface unit 206 is used for image transfer to PC by using a USB Interface.
Reference numeral 201 represents a wireless LAN module and is constituted of a wireless LAN unit 208 and an antenna 209. For image transfer using a wires LAN, the wireless LAN unit 208 receives image data read from the memory card 207 by the signal processing unit 203 and transfers the image data to a display apparatus side via the antenna 209.
There arises a problem of a consumption power when a wireless LAN is mounted on a portable apparatus such as a digital camera and a mobile phone. Most of wireless LAN cards of IEEE802.11b presently sold commercially have a consumption power of 800 mW or more for transmission and 600 mW or more for reception. This consumption power is a large load on a battery driven portable apparatus.
The consumption power can be lowered only by 80% even if a wireless LAN function is operated by limiting to a short distance and reducing a transmission power. Particularly, transfer from an image input apparatus such as a digital camera to an image display apparatus has a communication type that a transmission ratio occupies most of communications. Accordingly, there is a demand for means of wireless transmission with low consumption power.
[Non-Patent Document 1]
International Standard ISO/IEC 8802-11:1999 (E) ANSI/IEEE Std 802.11, 1999 Edition, Part 11:Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications